<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        header{
          font-size:12px; 
          width:1rem; 
        }
        @media all and (min-width:320px) and (max-width:568px){
            html{
                font-size:14px;
            }
            /* 5 */
        }
        @media all and (min-width:320px) and (max-width:568px){
            html{
                font-size:16px;
            }
        }
        @media all and (min-width:320px) and (max-width:568px){
            html{
                font-size:18px;
            }
        }
    </style>
</head>
<body>
    上节回顾
        媒体查询
        响应式

        dpr 
        px
        em   
        rem 
        vw
        vh      
        

    <header>头部</header>
</body>
</html>